EP1560390A1  Adaptive control of the bandwidth of a carrier recovery loop in presence of local oscillator phase noise  Google Patents
Adaptive control of the bandwidth of a carrier recovery loop in presence of local oscillator phase noise Download PDFInfo
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 EP1560390A1 EP1560390A1 EP20040290224 EP04290224A EP1560390A1 EP 1560390 A1 EP1560390 A1 EP 1560390A1 EP 20040290224 EP20040290224 EP 20040290224 EP 04290224 A EP04290224 A EP 04290224A EP 1560390 A1 EP1560390 A1 EP 1560390A1
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 European Patent Office
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 pll
 phase
 value
 phase error
 bandwidth
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 230000003044 adaptive Effects 0.000 title claims description 6
 230000000996 additive Effects 0.000 claims abstract description 9
 239000000654 additives Substances 0.000 claims abstract description 9
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 238000005457 optimization Methods 0.000 claims description 2
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 H—ELECTRICITY
 H03—BASIC ELECTRONIC CIRCUITRY
 H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION, OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
 H03L7/00—Automatic control of frequency or phase; Synchronisation
 H03L7/06—Automatic control of frequency or phase; Synchronisation using a reference signal applied to a frequency or phaselocked loop
 H03L7/08—Details of the phaselocked loop
 H03L7/085—Details of the phaselocked loop concerning mainly the frequency or phasedetection arrangement including the filtering or amplification of its output signal
 H03L7/093—Details of the phaselocked loop concerning mainly the frequency or phasedetection arrangement including the filtering or amplification of its output signal using special filtering or amplification characteristics in the loop

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/0014—Carrier regulation

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/0014—Carrier regulation
 H04L2027/0044—Control loops for carrier regulation
 H04L2027/0053—Closed loops
 H04L2027/0055—Closed loops single phase

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/0014—Carrier regulation
 H04L2027/0044—Control loops for carrier regulation
 H04L2027/0063—Elements of loops
 H04L2027/0069—Loop filters

 H—ELECTRICITY
 H04—ELECTRIC COMMUNICATION TECHNIQUE
 H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
 H04L27/00—Modulatedcarrier systems
 H04L27/0014—Carrier regulation
 H04L2027/0044—Control loops for carrier regulation
 H04L2027/0071—Control of loops
 H04L2027/0079—Switching between loops
 H04L2027/0081—Switching between loops between loops of different bandwidths
Abstract
Description
 The present invention relates to a radio transmission system. More particularly the invention relates to a circuit and a method for the control of the bandwidth of a carrier recovery loop in such radio transmission systems.
 In order to make efficient use of the microwave radio spectrum, stateofart digital pointtopoint radio systems employ highly bandwidthefficient modulation techniques. At present, certain known commercial systems employ 128 QAM signal constellation, nevertheless some 512 QAM prototypes are currently being fieldtested. For the sake of clarity it is to be noted that "QAM" stands for Quadrature Amplitude Modulation which is a well known amplitude modulation scheme in which amplitude modulation is performed by two separate signals of two sinusoidal carriers having the same amplitude and frequency but being in phase quadrature, the modulated signals are then added for transmission on a single channel.
 As the number of the points of the signal constellation grows, the system becomes more sensitive to all types of linear and nonlinear signal distortion. A particularly critical issue in bandwidthefficient QAM systems is the phase noise (PN) of the local oscillators (LO) which are used to convert the modulated signal from IF to RF and vice versa. Together with other noiselike degrading effects, LO  phase noise (LOPN) gives rise to a constant irreducible bit error rate (BER) independently from the power of the received signal.
 There are two ways known in the related art to face the problem of LOPN. The first known method is to achieve low noise local oscillators. However, the higher the radio frequency is, the more difficult it is to design and to produce a local oscillator with low phase noise. The second known solution is to choose a demodulation process which is somehow non sensitive towards PN.
 Consequently the first solution may become considerably expensive, taking into account in particular the fact that at present the radio frequencies that are being used are growing higher and higher in value (e.g. up to 90 GHz).
 Nevertheless, the second solution can be implemented with phase locked loop (PLL). In fact the PLL principle has been successfully used for decades for tracking the carrier phase. There is plenty of literature available to the public on PLL design and techniques. As a non limiting example reference is made to Floyd M. Gardner, "Phaselock Techniques", Second Edition, John Wiley & Sons.
 In the context of the above problem, the bandwidth of the PLL is the main parameter to take under consideration. The wider the bandwidth is, the better the demodulator ability becomes in order to track LOPN.
 However, there are two particular limits in achieving wide bandwidth for the PLL. One such limitation is the introduction of an additional source of phase noise which is due to the phase estimator error introduced by the modulated signal itself; this additional phase noise  that depends also from the quality of the received signal level and from the amount of the additive white gaussian noise (AWGN)  must, as much as possible, remain much lower than the original one. The second limit is the loop electrical delay. It turns out that a long loop electrical delay makes it impossible to go beyond a certain bandwidth, because the phase transfer function of the PLL approaches levels close to instability.
 From the above discussion it is clear that in order to reach an optimum design of the PLL, its bandwidth and its transfer function depend on the amount of LOPN as well as on the amount of the AWGN.
 The "standard" solution for achieving the above objectives would be to design a PLL with a fixed bandwidth as wide as possible. However, taking into account that the optimum bandwidth of the PLL depends on the signal to noise ratio of the received signal, then the choice of a fixed bandwidth cannot be considered an "optimum" solution because the signal to noise ratio can change depending on the circumstances.
 It is therefore an object of the present invention to overcome the above mentioned prior art drawbacks. This objective is achieved by the solution proposed by the present invention according to which by means of adaptively changing the transfer function of the PLL according to the signal to noise ratio (SNR) of the received signal, the BER of the system is improved. In fact, based on knowing the LOPN and estimating the amount of AWGN, it becomes possible to adaptively find the best bandwidth of the PLL, leading eventually to a substantial improvement in BER values.
 Accordingly it is an object of the present invention to provide a method for adaptive control of bandwidth of a carrier recovery loop in radio transmission systems, said system comprising a local oscillator and a phase locked loop  PLL  for tracking a carrier phase, the PLL having a transfer function, characterized in that a phase error in the PLL is adjusted to an optimum level by means of a loop filter incorporated in said PLL, said optimization of the phase error in the PLL comprising the steps of:
 a determining a first value relative to phase noise in the local oscillator;
 b estimating a second value relative to additive white gaussian noise in the PLL;
 c applying the said first and second values in a predetermined mathematical relationship representing the phase error value as a function of a transfer function value in said PLL; and
 d selecting a value of the PLL transfer function corresponding to an optimum phase error value according to said predetermined mathematical relationship.

 According to an aspect of the present invention, said optimum phase error value is a minimum value.
 Another object of the present invention is that of providing a system for performing adaptive control of bandwidth of a carrier recovery loop in radio transmission systems, comprising a local oscillator and a phase locked loop  PLL  for tracking a carrier phase, characterized in that the PLL comprises a loop filter incorporated therein for adjusting a phase error in the PLL according to the steps of the method above.
 These and further features and advantages of the invention are explained in more detail in the following description as well as in the claims with the aid of the accompanying drawing.
 The single figure is a block diagram showing a basic scheme of a PLL for use in the solution of the present invention.
 There are various known arrangements for PLLs, however, without loss of generality, it can be considered that the basic structure of a PLL can be represented in the block diagram of the figure wherein the PLL circuit comprises a phase detector 1, a loop filter 2, a voltage controlled oscillator 3 and an optional pure delay 4, the latter representing the undesired effect of a delay present in the loop, for example, due to a decision delay in a decision feedback loop. Furthermore the additional source of phase noise is modeled by µ(k) which, as mentioned further above, is due to the phase estimator error introduced by the modulated signal itself, depending also from the amount of AWGN.
 The input Φ(z) to the phase detector 1 is the difference between two signals, namely an input signal having a phase Θ_{i}(z) and an output signal fedback to the input of the phase detector 1 having a phase Θ_{o}(z).
 In the following, for the sake of simplicity, it is assumed that the system is already equalized and frequencysynchronized, and that both timing recovery and relative gain control have been established. Under the above assumptions, the baudrate samples r_{k} of the received signal, as it is know in the related art can be expressed as:
r_{k} =a_{k} ·e ^{+jϕi(k)} +n_{k}  Reference to the relationship (1) above, can be found among other available literature, in H. Meyr, M. Moeneclaey, S. A. Fetchel, "Digital Communication Receivers", John Wiley & Sons, 1998, page 341.
 The PLL open loop ztransform transfer function is shown below (equation 2). This is also a known and conventional formula in the techniques related to PLLs: in a simplified interpretation it may be said that this formula is the multiplication of the transfer function of each of the blocks shown in the figure:
L (z )=K ·F (z ) ·z ^{}^{N} 1 z ^{1}  In the above, L(z) represents the PLL open loop transfer function, K is the phase detector gain, i.e. the slope of the Scurve of the phase detector in the origin , F(z) is the transfer function of the loop filter and N represents the number of symbols in time delay. It is further to be noted that the open loop delay is expressed as NT, shown in the figure by reference numeral 4, wherein T represents symbol time.
 Reference to the above relationship (2) may be found in H. Meyr, M. Moeneclaey, S. A. Fetchel, "Digital Communication Receivers", John Wiley & Sons, 1998, pages 342343 and 97117.
 The solution according to the invention is based on designing a loop filter 2, which is capable of minimizing the phase error variance σ^{2} _{e} , subject to the following data and constrains:
 The power of the AWGN samples n_{k} is represented by σ^{2} _{n} ;
 the power spectral density of the phase noise ϕ _{i} (k) is represented by S _{ϕ}(f); where (f) is frequency. It is assumed that n_{k} and ϕ _{i} (k) are uncorrelated.
 K is the phase detector gain which is a fixed value but only known to belong to an interval K ∈ [K _{1},K _{2}], this parameter reflects the sensitivity of the Scurve of the phase detector to signal to noise ratio;


 Here again, reference to the above relationship (3) may be found in H. Meyr, M. Moeneclaey, S. A. Fetchel, "Digital Communication Receivers", John Wiley & Sons, 1998, pages 342343 and 97117.
 As can be noted from (3), increasing the loop bandwidth, the first term increases, related to AWGN contribution represented by σ^{2} _{n} while the second term decreases, related to phase noise contribution as represented by S_{ϕ} (f). This equation shows clearly the fact that the phase error, and consequently the optimum bandwidth are a function of phase noise and additive white gaussian noise.
 Therefore, knowing the phase noise of the local oscillator (LOPN) and estimating the amount of additive white gaussian noise, it is possible to adaptively find the best bandwidth that results in the minimization of equation (3), leading eventually to a great improvement in BER curves. This is done by obtaining the value of the transfer function of the PLL corresponding to the minimum value obtained for the phase error.
 It is to be noted that the mathematical relationship shown in equation (3) above is only an example provided in this description for a clear understanding of the solution provided by the invention. The scope of the invention is not to be construed as to be limited only to the above mathematical relationship. Those skilled in the related art would realize that other mathematical expressions can also be used in order to obtain the desired values of the PLL bandwidth according to the invention.
 The main advantage of the new solution is that, at every symbol time, the "optimum" PLL transfer function can be found and used, thus obtaining in this manner an improvement in BER.
 In order to estimate the power of the additive white gaussian noise σ^{2} _{n} various strategies exist from which, without loss of generality, we can mention the estimation of the Mean Square Error (MSE), that is a direct measurement of the amount of additive white gaussian noise.
 At this point the minimization of equation (3) can be done in different ways: in a preferred but not limiting embodiment, the optimum value for and, as a consequence, the optimum value for F(z), can be found by direct calculation or it can be found by making use of precalculated lookup tables.
 The cost function to be minimized can be depicted by means of other known expressions as a results of applying different models or different simplifications, within the scope of the solution proposed by the present invention, which is based on providing a radio transmission system with an adaptive bandwidth of the carrier recovery loop.
Claims (4)
 A method for adaptive control of bandwidth of a carrier recovery loop in radio transmission systems, said system comprising a local oscillator and a phase locked loop  PLL  for tracking a carrier phase, the PLL having a transfer function, characterized in that a phase error in the PLL is adjusted to an optimum level by means of a loop filter incorporated in said PLL, said optimization of the phase error in the PLL comprising the steps of:a determining a first value relative to phase noise in the local oscillator;b estimating a second value relative to additive white gaussian noise in the PLL;c applying the said first and second values in a predetermined mathematical relationship representing the phase error value as a function of a transfer function value in said PLL; andd selecting a value of the PLL transfer function corresponding to an optimum phase error value according to said predetermined mathematical relationship.
 The method of claim 1 wherein said optimum phase error value is a minimum value.
 The method of claim 1 or 2 wherein the predetermined mathematical relationship representing the phase error value as a function of a transfer function value in said PLL is expressed as follows: wherein:σ^{2} _{e} represents phase error variance;σ^{2} _{n} represents power value of additive white gaussian noise samples;Sϕ(f) represents power spectral density of the phase noise, where (f) is frequency;T represents symbol time;K represents linearized phase detector gain; andL(f) represents the transfer function of the PLL at frequency f.
 A system for performing adaptive control of bandwidth of a carrier recovery loop in radio transmission systems, comprising a local oscillator and a phase locked loop  PLL  for tracking a carrier phase, characterized in that the PLL comprises a loop filter incorporated therein for adjusting a phase error value in the PLL according to the steps of the method of any one of claims 1 to 3.
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EP20040290224 EP1560390A1 (en)  20040128  20040128  Adaptive control of the bandwidth of a carrier recovery loop in presence of local oscillator phase noise 
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EP20040290224 EP1560390A1 (en)  20040128  20040128  Adaptive control of the bandwidth of a carrier recovery loop in presence of local oscillator phase noise 
US11/043,205 US7443929B2 (en)  20040128  20050127  Method and circuit for adaptive control of the bandwidth of a carrier recovery loop in radio transmission systems 
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Cited By (2)
Publication number  Priority date  Publication date  Assignee  Title 

WO2010126844A1 (en) *  20090426  20101104  Qualcomm Incorporated  Jammer detection based adaptive pll bandwidth adjustment in fm receiver 
CN104283552A (en) *  20140506  20150114  浙江大学  Dynamic phaselocked loop bandwidth adjusting method for carrier extraction 
Families Citing this family (2)
Publication number  Priority date  Publication date  Assignee  Title 

US7702059B2 (en) *  20050209  20100420  Analog Devices, Inc.  Adaptable phase lock loop transfer function for digital video interface 
CN104521176B (en)  20120228  20180724  英特尔公司  The dynamic optimization of carrier auxiliary performance for communication system 
Citations (2)
Publication number  Priority date  Publication date  Assignee  Title 

EP1115237A1 (en) *  19980918  20010711  Kabushiki Kaisha Kenwood  Radio digital signal receiver 
WO2001080511A1 (en) *  20000417  20011025  Atmel Corporation  Phase noise and additive noise estimation in a qam carrier recovery circuit 
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AU472567B2 (en) *  19720120  19760527  National Aeronautics And Space Administration  Improved narrowband fm system for voice communications 
JPH06132816A (en) *  19920608  19940513  Sony Tektronix Corp  Phase lock loop circuit 
US5832041A (en) *  19941021  19981103  Philips Electronics North America Corporation  64 QAM signal constellation which is robust in the presence of phase noise and has decoding complexity 
US6577690B1 (en) *  19980625  20030610  Silicon Automation Systems Limited  Clock recovery in multicarrier transmission systems 
US6091303A (en) *  19990406  20000718  Ericsson Inc.  Method and apparatus for reducing oscillator noise by noisefeedforward 
US7912145B2 (en) *  20031215  20110322  Marvell World Trade Ltd.  Filter for a modulator and methods thereof 

2004
 20040128 EP EP20040290224 patent/EP1560390A1/en not_active Withdrawn

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 20050127 US US11/043,205 patent/US7443929B2/en not_active Expired  Fee Related
Patent Citations (2)
Publication number  Priority date  Publication date  Assignee  Title 

EP1115237A1 (en) *  19980918  20010711  Kabushiki Kaisha Kenwood  Radio digital signal receiver 
WO2001080511A1 (en) *  20000417  20011025  Atmel Corporation  Phase noise and additive noise estimation in a qam carrier recovery circuit 
NonPatent Citations (1)
Title 

OELLERMANN T ET AL: "Performance evaluation of a coded CDMA optical system" MILITARY COMMUNICATIONS CONFERENCE, 1996. MILCOM '96, CONFERENCE PROCEEDINGS, IEEE MCLEAN, VA, USA 2124 OCT. 1996, NEW YORK, NY, USA,IEEE, US, 21 October 1996 (19961021), pages 908912, XP010204032 ISBN: 0780336828 * 
Cited By (6)
Publication number  Priority date  Publication date  Assignee  Title 

WO2010126844A1 (en) *  20090426  20101104  Qualcomm Incorporated  Jammer detection based adaptive pll bandwidth adjustment in fm receiver 
CN102405600A (en) *  20090426  20120404  高通股份有限公司  Jammer detection based adaptive pll bandwidth adjustment in fm receiver 
US8437721B2 (en)  20090426  20130507  Qualcomm Incorporated  Jammer detection based adaptive PLL bandwidth adjustment in FM receiver 
CN102405600B (en) *  20090426  20141126  高通股份有限公司  Jammer detection based adaptive PLL bandwidth adjustment in FM receiver 
CN104283552A (en) *  20140506  20150114  浙江大学  Dynamic phaselocked loop bandwidth adjusting method for carrier extraction 
CN104283552B (en) *  20140506  20170728  浙江大学  A kind of method that PLL loop bandwidth for carrier extract is dynamically adjusted 
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US7443929B2 (en)  20081028 
US20050169418A1 (en)  20050804 
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